You are only allowed to file Married Filing Jointly (MFJ) or Married Filing Separately (MFS) when you are considered legally married at the end of a tax year. 

It is advised to file MFJ because MFS normally will put you in a higher tax bracket, and you will need to include one-half of your spouses income.

You do not have to wait to file the 2018 tax return as MFJ.  The adjusted gross income verification, that I am assuming you are waiting for, is only for electronic tax returns. You can print and submit a paper return to the taxing agencies.